1. Field of the Invention
The present invention relates generally to inertial type voice coil actuators capable of converting energy between electrical and mechanical form and, more particularly, to an inertial type voice coil actuator that utilizes radially polarized biasing magnets and a multicomponent suspension for alignment of the moving coil.
2. Description of the Prior Art
Inertial voice coil actuators have been used in the past to acoustically stimulate semi-rigid structures to radiate sound. In this application, voice coil actuators have been attached to structures that are relatively large to act as a soundboard such as a wall in a room, where the wall of the room, when acoustically driven radiates sound. As is well known in the art, the force generated by an electrodynamic transducer is a product of the current, I, length of coil wire, L and flux density, B so that F=iL{circle around (×)}B. The length of the coil wire that is within the annular magnetic gap is defined as the length, L. This force is what creates the movement of the coil and subsequently generates sound.
These inertial type voice coil transducers are built upon magnetic circuit designs that have classically been used for conventional cone type loudspeakers and not optimized for driving soundboard type structures. These voice coil actuators often require the use of an external housing to support the heavy magnet assembly relative to the voice coil. The voice coil is in communication with the external housing at a location coincident with an acoustic output system that permits the transducer housing to be mechanically attached to a soundboard.
Loudspeaker motors such as used in the past comprise a magnet circuit assembly including a permanent annular magnet, polarized in the axial direction, and sandwiched between two magnetizable plates. One of the plates carries a cylindrical post that extends through a central space defined by the annular magnet, generally referred to as a cylindrical pole piece. The other plate has an annular opening, somewhat larger than the diameter of the pole piece, such that an annular magnetic gap is formed between the post and the inner edge of the associated annular plate. The height of the gap is formed by the thickness of the annular plate having the annular opening.
The basic architecture of the loudspeaker motor design is based upon low magnetic energy magnets, typically comprised of ceramic materials. In order for sufficient magnetic flux to be generated within the annular magnetic gap, the annular magnet must be very large relative to the other components. Some manufacturers have utilized higher energy rare earth based magnets such as Neodymium, but this magnetic architecture is not optimized for the characteristics of these magnets.
Voice coil actuators have a moveable voice coil disposed within the annular magnetic gap. For speakers that use a large body such as a wall to generate sound, the coil has a suspension system that typically utilizes an external housing to which the annular magnet and magnetizable plates are also attached. The external housing provides radial stiffness and axial compliance to the coil. The moving coil has a first end fixedly secured to a radially central portion of the inner surface of the external housing wall. A mounting screw secured to an exterior well portion of the exterior housing may be attached to the wall.
A number of inventions for voice coil actuators have been patented which disclose the aforementioned factors, among them U.S. Pat. No. 2,341,275 to Holland for Sound Reproducing Instrument; U.S. Pat. No. 3,609,253 for Loudspeaker with Improved Voice Coil Suspension; U.S. Pat. No. 3,728,497 to Komatsu for Dynamic Loudspeaker Using Wall as Diaphragm; U.S. Pat. No. 4,297,537 to Babb for Dynamic Loudspeaker; U.S. Pat. No. 4,951,270 for Audio Transducer Apparatus; U.S. Pat. No. 5,335,284 to Lemons for Coneless, No-Moving-Parts Speaker; and U.S. Pat. No. 5,473,700 Fenner, Jr. for High Gain Transducer.
In practice, the annular magnet, magnetizable plates, external housing and structural attachment point comprise a system that is large and heavy relative to the total dynamic force the actuator is capable of generating. If the external housing is mounted on a vertical facing surface e.g. a wall, large bending moments are placed on the structural attachment point and the housing must accommodate these moments without translating them to the coil.
These types of electrodynamic transducers are plagued with well known problems of low power handling, limited frequency response, high levels of sound distortion, substantial size and mass, mechanical complexity and high production costs.
Recent innovations include magnetic materials that have produced magnets with substantially greater magnetic energy than ceramic magnets. These magnets have necessitated the redesign of the magnetic circuit to take advantage of the higher magnetizing flux while reducing the volume of the magnet material consumed, thus reducing its size while simultaneously increasing its force density per unit volume. However, these prior art voice coil actuators are not typically designed with suspension systems adequate for actuators driving relatively large structures such as walls.
U.S. Pat. No. 4,297,537 to Babb for Dynamic Loudspeaker describes an antifriction bearing which adjoins the voice coil and slidably moves on the cylindrical pole piece providing high radial stiffness and essentially infinite compliance in the axial motion of the voice coil. This patent describes a magnetic circuit with an annular magnet where the voice coil is driving a conventional cone speaker. It does not utilize a large body for sound generation nor is it designed to be vertically mounted.
U.S. Pat. No. 5,335,287 to Lewis for Loudspeaker Utilizing Magnetic Liquid Suspension of the Voice Coil discloses a method of using a viscous magnetic fluid suspension for the voice coil in lieu of a corrugated disk suspension. However, use of such fluid can result in internal pressure build-ups or subatmospheric conditions within the magnetic gaps. U.S. Pat. No. 5,335,287 solves that problem by including a fairly sophisticated venting system, however, the system is expensive to manufacture and the speaker disclosed is of the traditional cone type without adaptation to large sound bodies. No means is provided to minimize flux leakage.
Increasingly, high fidelity audio recordings are being made where the upper frequency range is over one (1) octave higher than normal human hearing at 20 kHz. Accurate reproduction of these frequencies is often not addressed or is only poorly accomplished by earlier speaker systems.
It is therefore an object of the present invention to provide a novel voice coil actuator with a high force density. It is a second object of the present invention to minimize flux leakage while providing a smaller and more efficient device for driving relatively large structures. A third objective of the invention is to minimize sound distortion by providing a multi component voice coil suspension system. A fourth objective of the invention is to provide an inertial voice coil actuator equipped with a simple mounting system for transducing sound to a soundboard.
A fifth objective is to provide an inertial voice coil actuator equipped with means to quickly and removably affix the voice coil actuator to various surfaces without the use of adhesive bonding between the output disk and the soundboard and without the need for tools thereby minimizing assembly and repair time.
A sixth objective is to provide an inertial voice coil actuator that may be installed intra-wall without loss in sound quality.
It is another object of the present invention to further provide a means to couple the voice coil actuator with a soundboard utilizing controlled pressure where the voice coil actuator is not exposed, but is installed within a wall.
Yet another object is to provide a method of enhancing the system for extended high frequency response.
It is further a feature of the present invention to provide means to supply a signal and current to the voice coil actuator through a retainer where contacts are configured to maintain their electrical connection even with slight axial translation of the voice coil actuator.
It is a final feature of the present invention to govern the placement of the voice coil actuator between the vertical studs of a wall in order to diminish resonate frequencies of the soundboard.